The purpose of this project was to elucidate the mechanisms underlying cellular remodeling in different cell types in response to different agents. The response of L-cells to cyclic AMP and changes of shape of Fc receptor bearing lymphocytes in response to immobilized antigen antibody complexes have been studied. Methods employed included time-lapse photography, electron microscopy, treatment with various drugs, and in the case of L-cells, electrophysiology. The roles of various cellular organelles in shape changes have been outlined and some suggestive evidence as to some control mechanisms has been obtained. There are differences between cell types in the detailed patterns of changes involved in the metamorphosis from one shape to another.